This invention relates generally to an improved regulating scheme for stabilizing the operation of a current fed induction motor drive system and, more particularly, to apparatus for providing consistent torque response over a variable excitation range.
An induction motor typically comprises a squirrel cage or wound rotor that is mounted in a stator having windings connected to a suitable source of excitation. Excitation of the stator windings creates a magnetic flux across the stator-rotor air-gap of the motor and the current induced in the rotor interacts with the air-gap flux to produce an electromagnetic force or torque tending to move the rotor relative to the stator. The amount of torque developed by the motor is often expressed in terms of the magnitude of the air-gap flux and the slip frequency between stator and rotor. The effective slip frequency by definition is the difference between the frequency of the flux wave on the air gap and the equivalent electrical frequency at which the motor shaft is rotating (i.e., motor speed). Where such a motor is reqired to run at variable speeds with variable loads and in both forward and reverse directions, as in the case of traction motors for electrically propelled vehicles, the stator windings are advantageously supplied with polyphase a-c power which is so conditioned that the frequency as well as the amplitude of the stator excitation are adjustable as desired and the phase sequence is reversible.
A review of control systems for current fed induction motor drives is provided in U.S. Pat. No. 4,088,934 issued for J. D. D'Atre, T. A. Lipo, and A. B. Plunkett and assigned to General Electric Company. That patent teaches stabilizing a current fed induction motor drive system by controlling the excitation source of the motor as a function of the actual phase angle between the air gap flux and the stator current in the motor. The control system varies the frequency of the current supplied to the stator so as to regulate the phase angle to a desired value. In addition, the magnitude of air-gap flux is directly monitored and the magnitude of excitation applied to the stator is controlled so as to regulate the magnitude of air-gap flux to a desired value.
The family of characteristic curves of torque versus slip for an a-c motor illustrates a changing slope with a changing flux magnitude. As the level of flux increases, the slope increases in a proportional degree. The net result is that at a high flux level, a given change in motor slip frequency causes a greater variation in motor torque than would occur for the same change in slip frequency at a lower flux level. In order for the control system to consistently operate the motor at its most advantageous operating characteristic for any commanded torque output, the system must be responsive to this variation in gain of the motor with changes in flux magnitude.